Accompanying recent popularization of computers, an inkjet printer is widely used for printing letters or an image on paper, film, cloth or the like not only in offices but also at homes.
The inkjet recording method includes a system of jetting out a liquid droplet by applying a pressure using a piezoelectric element, a system of jetting out a liquid droplet by generating bubbles in an ink under heat, a system using an ultrasonic wave, and a system of jetting out a liquid droplet by suction using an electrostatic force. With respect to the ink composition for such inkjet recording, an aqueous ink, an oily ink or a solid (fusion-type) ink is used. Among these inks, an aqueous ink is predominating in view of production, handleability, odor, safety and the like.
The coloring agent used in such an ink for inkjet recording is required that the solubility in a solvent is high, high-density recording can be performed, good color and excellent fastness to light, heat, air, water and chemicals are exhibited, good fixing on an image-receiving material and less bleeding are ensured, an ink having excellent storability is provided, and the coloring agent has high purity and no toxicity and is available at a low cost. However, it is very difficult to find out a coloring agent satisfying these requirements in a high level. Various dyes and pigments for inkjet use have been already proposed and actually used, however, a coloring agent satisfying all requirements is not yet found at present. Conventionally well-known dyes and pigments having a color index (C.I.) number can hardly satisfy both color hue and fastness required of the ink for inkjet recording.
As the inkjet black dye, food black-type dyes, naphthol-base direct azo dyes, acid azo dyes and the like are heretofore widely known.
Representative examples of the food black-type dye include C.I. Food Black 1 and C.I. Food Black 2 and techniques of using these dyes for inkjet black ink are described in JP-A-2-36276 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-2-233782 and JP-A-2-233783.
As for the acid azo dye, techniques of using C.I. Acid Black 2, C.I. Acid Black 31, C.I. Acid Black 52, C.I. Acid Black 140, C.I. Acid Black 187 or the like for inkjet black ink are described in JP-A-60-108481, JP-A-2-36277 and JP-A-2-36278. Also, as for the direct azo dye, techniques of using C.I. Direct Black 9, C.I. Direct Black 17, C.I. Direct Black 38, C.I. Direct Black 51, C.I. Direct Black 60, C.I. Direct Black 102, C.I. Direct Black 107, C.I. Direct Black 122, C.I. Direct Black 142, C.I. Direct Black 154, C.I. Direct Black 168 or the like for the inkjet black ink are described in JP-A-56-139568, JP-A-61-285275 and JP-A-3-106974.
Usually, the black color can be hardly covered by a sole dye and a short-wave dye is preferably used in combination.
As the short-wave dye, similarly, direct azo dyes, acid azo dyes and the like are widely known and techniques of using C.I. Acid Yellow 17, C.I. Acid Yellow 23, C.I. Acid Yellow 49, C.I. Acid Yellow 194 or the like or using C.I. Direct Yellow 86, C.I. Direct Yellow 120, C.I. Direct Yellow 132, C.I. Direct Yellow 144 or the like for inkjet black ink are described in JP-A-7-97541, WO97/16496, JP-A-10-158560 and JP-A-11-12514. In these related arts, the above-described properties required of the inkjet ink are not yet fully satisfied and more improvements are demanded. Particularly, the aqueous black ink is demanded to be improved in the color hue, namely, non-loosening of black, and the fastness.
The present inventors have made studies on the inkjet ink using a dye and found the followings. The aqueous black ink has a problem in that the image durability is low. Also, unlike carbon black, a black dye capable of giving by itself a black image such as letter having high printing quality is not known and as described above, a plurality of dyes differing in the absorption spectrum should be used in combination. However, if a plurality of dyes differing in the absorption spectrum are used, the discoloration rate differs depending on the dye mixed and when a dye having a broad absorption property is changed in the absorption waveform due to discoloration, the quality of image including black is seriously impaired.
Furthermore, although the spectrum must be supplemented by various dyes so as to cover the broad region of black and obtain a good black ink, unless all dyes have high weather resistance, the balance of black is lost in aging.
When the inkjet recorded image has fastness and the image quality can be maintained during the long-term storage of image, the utility value of inkjet recording system is enhanced and therefore, it is keenly demanded to solve the above-described problems.